Stability-enhancing underlayer for exchange-coupled magnetic structures, magnetoresistive sensors, and magnetic disk drive systems

ABSTRACT

An exchange-coupled magnetic structure includes a ferromagnetic layer, a coercive ferrite layer, such as cobalt-ferrite, for biasing the magnetization of the ferromagnetic layer, and an oxide underlayer, such as cobalt-oxide, in proximity to the coercive ferrite layer. The oxide underlayer has a lattice structure of either rock salt or a spinel and exhibits no magnetic moment at room temperature. The underlayer affects the structure of the coercive ferrite layer and therefore its magnetic properties, providing increased coercivity and enhanced thermal stability. As a result, the coercive ferrite layer is thermally stable at much smaller thicknesses than without the underlayer. The exchange-coupled structure is used in spin valve and magnetic tunnel junction magnetoresistive sensors in read heads of magnetic disk drive systems. Because the coercive ferrite layer can be made as thin as 1 nm while remaining thermally stable, the sensor satisfies the narrow gap requirements of high recording density systems.

RELATED APPLICATION

This is a divisional application of the application bearing Ser. No.09/841,942 filed Apr. 24, 2001 which has been allowed.

FIELD OF THE INVENTION

This invention relates generally to magnetic devices such as spin valvemagnetoresistive (MR) sensors and magnetic tunnel junctions. Moreparticularly, it relates to exchange-coupled magnetic structurescontaining underlayers that enhance the stability of coercive ferritelayers used to bias the magnetic moment of adjacent ferromagneticlayers.

BACKGROUND ART

Computer systems generally use auxiliary memory storage devices havingmedia on which data can be written and from which data can be read forlater use. A direct access storage device (disk drive) incorporatingrotating magnetic disks is commonly used for storing data in magneticform on the disk surfaces. Data are recorded on concentric, radiallyspaced tracks on the disk surfaces. Magnetic heads including readsensors are then used to read data from the tracks.

A magnetoresistive (MR) sensor detects a magnetic field through thechange of its resistance as a function of the strength and direction ofthe magnetic flux being sensed by the MR layer. Most current MR sensorsare based on the giant magnetoresistive (GMR) effect. In GMR sensors,the resistance of the MR sensing layer varies as a function of thespin-dependent transmission of conduction electrons between magneticlayers separated by an electrically conductive non-magnetic spacer layerand the accompanying spin-dependent scattering that takes place at theinterface of the magnetic and non-magnetic layers and within themagnetic layers. The external magnetic field causes a variation in therelative orientation of the magnetic moments (magnetizations) of themagnetic layers, thereby affecting the spin-dependent transmission ofconduction electrons and the measurable device resistance.

GMR sensors using at least two layers of ferromagnetic materialseparated by a layer of non-magnetic electrically conductive materialare generally referred to as spin valve MR sensors. In a spin valvesensor, one of the ferromagnetic layers, referred to as the pinnedlayer, has its magnetization pinned by exchange coupling with anantiferromagnetic (AFM) layer. The magnetization of the otherferromagnetic layer, referred to as the free layer, however, is notfixed and is free to rotate in response to the field from the recordedmagnetic medium, i.e., the signal field. In spin valve sensors, theresistance varies as the cosine of the angle between the magnetizationof the pinned layer and the magnetization of the free layer. Recordeddata can be read from a magnetic medium because the external magneticfield from the recorded magnetic medium causes a change in the directionof magnetization in the free layer, which in turn causes a change inresistance of the sensor and a corresponding change in the sensedcurrent or voltage. The sensor is in a low resistive state if the twomagnetizations are parallel and a high resistive state if the twomagnetizations are antiparallel.

Conventional spin valve MR sensors take on several forms, includingsimple, antiparallel-pinned, and dual. A simple spin valve MR sensor 100is shown in cross section in FIG. 1. A free ferromagnetic layer 105 isseparated from a pinned ferromagnetic layer 103 by a non-magnetic,electrically conducting spacer layer 104. The magnetization of pinnedlayer 103, in this case into the paper, is fixed or pinned throughexchange coupling with a pinning layer 102, which is typically an AFMmaterial with a high Neel temperature such as NiO. A hard biasing layer111 sets the magnetization of free layer 105, in this case in the planeof the paper, perpendicular to the magnetization of pinned layer 103,through magnetostatic coupling. A set of leads 112 contact pinned layer103 to supply sense current to the device; the sense current is measuredto detect the varying device resistance induced by the external magneticfield. One or more underlayers 101, such as tantalum, zirconium,nickel-iron, or alumina, are provided to control growth of thesuccessive layers, and the device is typically terminated by a cappinglayer 106 to prevent corrosion.

In an antiparallel (AP)-pinned spin valve MR sensor, the pinned layer isreplaced by a laminated structure that acts as an artificialferrimagnet. FIG. 2 is a cross-sectional view of an AP-pinned spin valveMR sensor 120 with underlayers 121, a pinning layer 122, a conductivelayer 126, a free layer 127, and a capping layer 128 that are identicalto layers 101, 102, 104, 105, and 106, respectively, of FIG. 1. However,pinned layer 103 of simple spin valve 100 is replaced by anantiparallel-pinned structure 129 consisting of a ferromagnetic pinnedlayer 123, an antiferromagnetic coupling layer 124, and a ferromagneticreference layer 125. The magnetizations of pinned layer 123 andreference layer 125 are coupled antiparallel to each other throughcoupling layer 124 and are perpendicular to the magnetization of freelayer 127. AP-pinned spin valves are advantageous because the netmagnetic moment of AP-pinned structure 129, the difference between thetwo antiparallel moments of the component ferromagnetic layers, can bevaried independently of the thickness of pinned layer 123. Thus it ispossible to balance the overall moment of the spin-valve while choosinga preferred thickness for the pinned layer 123. Furthermore, AP-pinnedspin valves also exhibit a greatly enhanced stability in comparison withsimple spin valves, since the coupling between pinned layer 123 andreference layer 125 is quite high.

A dual spin valve is a simple or AP-pinned spin valve with a second setof conductive, pinned, and antiferromagnetic layers deposited on top ofit. Each of the pinned layers can be either a single ferromagnetic layeror an artificial ferrimagnetic layer as described above. Dual spinvalves exhibit an enhanced sensitivity, but are much thicker andtypically show a lower resistance than the previously described simpleand AP-pinned spin valves.

As a real recording densities in magnetic media continue to increase,smaller magnetoresistive sensors with higher signals are required. MRsignals are measured as ΔR/R, the percent change in device resistance asthe ferromagnetic layer magnetizations switch between parallel andantiparallel. Specifically, as densities approach 100 Gbit/in², the gapbetween the shields of the read head, in which the sensor is positioned,must decrease from current thickness of 0.1 μm to between 50 and 70 nm.Smaller sensors require thinner layers, which tend to produce lowersignals. NiO pinning layers are unsatisfactory in these thicknessregimes because of their low magnetic anisotropy energy, which leads toa weak pinning field and a high critical layer thickness. The lowordering temperature of NiO also causes thermally unstable pinning. As asolution to this problem, cobalt-ferrite pinning layers were introducedin co-pending U.S. patent application Ser. No. 09/755,556, filed Jan. 4,2001, (issued as U.S. Pat. No. 6,721,144) is herein incorporated byreference.

While cobalt-ferrite provides a number of advantages over NiO and otherstandard AFM pinning layer materials, it also introduces two problems.First, coercive ferrites are thermally unstable in the thickness regimeof approximately 30 nm or less, which is required for 50-nm gap sensors.Second, unlike AFM pinning layers, ferrites exhibit a substantialmagnetic moment that contributes to the overall device moment, making itdifficult to balance the device moment as required for stable andconsistent operation. Thicker layers contribute a greater moment, and soreducing the pinning layer thickness while maintaining thermal stabilitywould address both problems.

1 Spin valves containing an oxidized iron layer inserted at the pinnedlayer/NiO pinning layer interface are disclosed in R. F. C. Farrow etal., “Enhanced blocking temperature in NiO spin valves: Role of cubicspinel ferrite layer between pinned layer and NiO,” Applied PhysicsLetters, 77(8), 1191-1193 (2000). The iron oxide layer is converted to acubic spinel nickel-ferrite (Ni_(0.8)Fe_(2.2)O₄) by solid-state reactionwith the NiO layer during annealing. The exchange bias field originatesfrom both the NiO pinning layer and the nickel-ferrite layer.Nickel-ferrite has a relatively low coercivity; for example, it isgenerally not possible to grow nickel-ferrite with coercivities of 1kOe. While nickel-ferrite/NiO spin valves display increased blockingtemperature (temperature at which the exchange field drops to zero) andimproved thermal stability, they cannot fit within a 50 nm sensor gap.

1 There is still a need, therefore, for an improved exchange-coupledmagnetic structure that uses coercive ferrite pinning layers and remainsthermally stable when reduced to the thicknesses required for magneticread heads.

OBJECTS AND ADVANTAGES

Accordingly, it is a primary object of the present invention to providea spin valve magnetoresistive sensor that remains thermally stable whenthinned sufficiently to fit within a 50-nm gap.

It is a further object of the invention to provide an underlayer forcoercive ferrite pinning layers that increases the coercivity andthermal stability of the pinning layers of a given thickness.

It is an additional object of the invention to provide an underlayer forcoercive ferrite pinning layers that allows the pinning layer to bethinned to as low as 1 nm.

It is another object of the invention to provide a MR sensor that can befabricated to desired thicknesses using standard fabrication techniques.

SUMMARY

These objects and advantages are attained by a magnetoresistive (MR)sensor having at least three layers: a ferromagnetic layer, a coerciveferrite layer for biasing the magnetization of the ferromagnetic layer,and an oxide underlayer in proximity to the coercive ferrite layer. Thestructure of the underlayer favorably affects the growth and structureof the coercive ferrite layer and therefore its magnetic properties,providing enhanced stability at smaller thickness.

The oxide underlayer, which is preferably sputter, ion-beam, pulsedlaser, or chemical vapor deposited, has either a rock salt or spinellattice structure and exhibits no magnetic moment at room temperature;that is, the underlayer is either diamagnetic, paramagnetic, orantiferromagnetic. For example, the oxide underlayer can be ZO_(1+x),where −0.3≦x≦0.3 and Z is Co, Ni, Mg, Mn, or one of their alloys.Alternatively, the underlayer can be Co₃O₄, MgAl₂O₄, or an alloy ofCo₃O₄ or MgAl₂O₄. The coercive ferrite layer, which has a thickness ofbetween 1 and 30 nm, is either Co_(x)Fe_(3−x)O₄, where 0≦x≦1.5(preferably x=1), SrFe₁₂O₁₉, BaFe₁₂O₁₉ or one of their alloys with Si,Ti, Mg, Al, Mo, Os, Re, Ru, or W. The underlayer is either in directcontact with the coercive ferrite layer or separated from it by anintermediate layer that preserves the structural effect of theunderlayer on the coercive ferrite layer.

The MR sensor also contains upper and lower shields defining a gap ofwidth less than 50 nm. In one embodiment, the MR sensor is a spin valveMR sensor, the ferromagnetic layer is a pinned layer, and the coerciveferrite layer is a pinning layer that pins the magnetization of thepinned layer. In another embodiment, the ferromagnetic layer is a freelayer and the coercive ferrite layer is a hard-bias layer that biasesthe magnetization of the free layer. For example, the MR sensor can be asimple, antiparallel-pinned, or dual spin valve MR sensor or a magnetictunnel junction MR sensor. In a third embodiment, the ferromagneticlayer is a free layer of a spin valve MR sensor and the coercive ferritelayer is an in-stack biasing layer that biases the magnetization of thefree layer.

The present invention also provides an exchange-coupled magneticstructure containing a ferromagnetic layer, a coercive ferrite layer,and an oxide underlayer in proximity to the coercive ferrite layer. Anexchange bias field from the coercive ferrite layer biases themagnetization of the ferromagnetic layer. The oxide underlayer haseither a rock salt type or spinel lattice structure and exhibits nomagnetization at room temperature. For example, the ferromagnetic layeris a free layer whose magnetization is biased by the coercive ferritelayer, and the structure also includes a pinned ferromagnetic layer andan insulating barrier layer separating the pinned ferromagnetic layerfrom the free layer. In this case, the structure is a magnetic tunneljunction.

Also provided is a magnetic disk drive system containing a magneticrecording disk, a magnetoresistive read/write head containing a MRsensor, an actuator for moving the read/write head across the magneticrecording disk, and a motor for rotating the magnetic recording diskrelative to the read/write head. The MR sensor contains at least oneexchange-coupled magnetic structure of the present invention asdescribed above.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional view of a simple spin valve MR sensor of theprior art.

FIG. 2 is a cross-sectional view of an AP-pinned spin valve MR sensor ofthe prior art.

FIG. 3A is a cross-sectional view of a simple spin valve MR sensor ofthe present invention. FIG. 3B shows the spin valve MR sensor of FIG. 3Awith an optional intermediate layer deposited between an underlayer anda coercive ferrite layer.

FIGS. 4A-4C are TEM cross-sections of AP-pinned spin-valves without anunderlayer (FIG. 4A) and with a 17 nm Co-oxide underlayer, at medium(FIG. 4B) and high (FIG. 4C) resolutions.

FIG. 5 is a radial X-ray diffraction spectrum of polycrystalline CoO andCo₃O₄ grown on Si (100).

FIG. 6 is a plot of coercivity versus underlayer thickness for simplespin valves containing CoO and CO₃3O₄ underlayers in the followingstructure: underlayer (x nm)/CoFe₂O₄ (25 nm)/Co_(0.84)Fe_(0.16) (2nm)/Cu (2.5 nm)/Co_(0.84)Fe_(0.16) (2 nm)/Ni₈₀Fe₂₀ (4 nm)/Ru (3 nm).

FIG. 7 is a plot of magnetoresistive ratio (ΔR/R) versus underlayerthickness for the simple spin valves of FIG. 6. Co₃O₄

FIG. 8 is a plot of magnetoresistance versus applied magnetic field Hfor an AP-pinned spin valve with a Co₃O₄ underlayer and a structure ofCo₃O₄ (10 nm)/CoFe₂O₄ (10 nm)/CO_(0.84)FeO_(0.16) (2 nm)/Ru (0.8nm)/Co_(0.84)Fe_(0.16) (1.5 nm)/Cu (2.5 nm)/Co_(0.84)Fe_(0.16) (0.5nm)/NiFe (4 nm)/Ru (3 nm). The critical fields H⁺ and H⁻ and thecoercivity H_(c) are indicated in the figure.

FIG. 9 is a plot of the critical magnetic fields H⁻ and H⁺ versustemperature for the AP-pinned spin valve of FIG. 8.

FIG. 10 shows a minor loop (magnetoresistance versus pinning field) forthe AP-pinned spin valve of FIG. 8 at 130.degree. C.

FIG. 11 is a series of magnetoresistance curves for AP-pinned spinvalves with different CoO underlayer thicknesses and structures of CoO(x nm)/CoFe₂O₄ (3 nm)/Co (3 nm)/Ru (0.7 nm)/Co (2 nm)/Cu (2.5 nm)/Co(0.5 nm)/Ni₈₀Fe₂₀ (4 nm)/Ru (3 nm).

FIG. 12 shows minor loops obtained after successively applying magneticfields of increasing magnitude in the transverse direction to anAP-pinned spin valve with a structure of CoO (17 nm)/CoFe₂O₄ (3 nm)/Co(2.4 nm)/Ru (0.7 nm)/Co (2 nm)/Cu (2.5 nm)/Co (0.5 nm)/Ni₈₀Fe₂₀ (4nm)/Ru (3 nm).

FIGS. 13A-13B are plots of magnetic moment per area and coercivity,respectively, versus film thickness of CoFe₂O₄ films grown ontoSi/SiO_(x) alone and onto CoO (20 nm) on Si/SiO_(x).

FIG. 14 is a cross-sectional view of a magnetic tunnel junction MRsensor with hard biasing performed according to the present invention.

FIG. 15 is a cross-sectional view of a currentperpendicular-to-the-plane (CPP) MR sensor with hard biasing performedaccording to the present invention.

FIG. 16 is a cross-sectional view of a spin valve MR sensor within-stack biasing performed according to the present invention.

FIG. 17 is a schematic diagram of a disk drive system of the presentinvention including an exchange-coupled magnetic structure.

DETAILED DESCRIPTION

Although the following detailed description contains many specifics forthe purposes of illustration, anyone of ordinary skill in the art willappreciate that many variations and alterations to the following detailsare within the scope of the invention. Accordingly, the followingembodiments of the invention are set forth without any loss ofgenerality to, and without imposing limitations upon, the claimedinvention.

The present invention provides an exchange-coupled magnetic structure,magnetoresistive (MR) sensors incorporating the structure, and amagnetic disk drive system including a magnetic read/write headcontaining the MR sensor. All of the devices contain oxide underlayers,e.g., CoO or Co₃O₄, on which a coercive ferrite biasing or pinning layeris grown. The magnetic properties of a magnetic thin film can be alteredsignificantly with the proper choice of underlayer, as underlayers areknown to influence crystalline growth direction and growth mode ofsubsequent layers. Because magnetic properties are dependent onstructural properties, the underlayers of the present invention affectcoercivity, magnetic moment, and squareness of subsequent layers ofmagnetic thin films, allowing the biasing layer to be thinnedconsiderably. The resulting underlayer/biasing layer bilayer is ofcomparable or lower thickness than a coercive ferrite layer without anunderlayer, is stable at higher temperatures, and contributes a lowermoment to the device. All embodiments of the invention discussed belowcontain an oxide underlayer, a coercive ferrite layer, and aferromagnetic layer whose magnetization is biased by the coerciveferrite layer.

First Embodiment—Spin Valve Magnetoresistive Sensor

FIG. 3A illustrates the underlayer of the present invention in a simplespin valve magnetoresistive sensor 130, shown in cross section. Sensor130 contains substantially the same layer structure as prior art spinvalve MR sensor 100, including pinned layer 103, conductive layer 104,free layer 105, capping layer 106, hard-bias material 111, and leads112. It also contains a coercive ferrite pinning layer 132 of thicknesst_(p) and an oxide underlayer 134 of thickness t_(u). As explainedbelow, oxide underlayer 134 directs the growth of coercive ferrite layer132, thereby increasing its coercivity and thermal stability. As aresult, coercive ferrite layer 132 can be made very thin; t_(p) isbetween 1 and 30 nm; t_(u) is between 1 and 30 nm. Sensor 130 fits in agap of thickness t_(gap) between a top shield 136 and a bottom shield138 of a magnetic read head. Sensor 130 also typically contains aninsulating underlayer 135 such as alumina.

Underlayer 134 has a crystalline structure either of rock salt (e.g.,CoO) or of a spinel (normal, inverse, or mixed) that exhibits nomagnetic moment at room temperature (e.g., Co₃O₄). Such underlayers werefound by the present inventors to enhance significantly the thermalstability of spin valves incorporating cobalt ferrite, a coerciveferrite, as a pinning layer 132 and t_(p) can be reduced to as low as 1nm while maintaining sufficient thermal stability of pinning layer 132.Since coercive ferrites are ferrimagnets, they exhibit a moment. Thereduced thickness of coercive ferrite layer 132 therefore also makes iteasier to balance the overall moment of spin valve 130. The thermalstability of the inventive coercive ferrite-based sensors with suitableoxide underlayers (e.g., 3 nm CoFe₂O₄ and 17 nm CoO) is also superior topresent PtMn-based sensors of comparable thickness. Furthermore, becauseoxide underlayer 134 is insulating, the thickness of Al₂O₃ underlayer135 can also be reduced, contributing to the compatibility of sensor 130with a 50 nm gap.

In spin valve 130, underlayer 134 contacts coercive ferrite layer 132directly. Alternatively, as shown in FIG. 3B an intermediate layer 139can be deposited between underlayer 134 and coercive ferrite layer 132.The intermediate layer 139 is a nanolayer that preserves the effect ofunderlayer 134 on the structure of coercive ferrite layer 132.Nanolayers are commonly used to improve the growth mode of subsequentlayers. For example, a nanolayer of a material with low surface freeenergy can be deposited onto undedayer 134 in order to smooth theinterface for the subsequent deposition of coercive ferrite layer 132.The structure of the nanolayer is determined by the structure ofunderlayer 134 and therefore directs the structure of pinning layer 132.

While spin valve magnetoresistive sensors are particularly usefulapplications of the present invention, the inventive underlayers may beused in any suitable exchange-coupled magnetic structure. The inventiveoxide underlayers of coercive ferrites affect the structural andresultant magnetic properties of the coercive ferrites, enhancing theirperformance in biasing the magnetic moment (magnetization) of a nearbyferromagnetic layer of the structure for a variety of applications.Additional applications are discussed below.

As used herein, coercive ferrites, for example as pinning layer 132, areferrites capable of being deposited with coercivities of at least 1 kOe.Examples include cobalt-ferrite, strontium-ferrite, and barium-ferriteand their alloys with Si, Ti, Mg, Al, Mo, Os, Re, Ru, or W. Note thatnickel-ferrite typically can only be grown with coercivities much lessthan 1 kOe and is therefore not a coercive ferrite. As used herein,cobalt-ferrites include any material with a formula Co_(x)Fe_(3−x)O₄,where 0≦x≦1.5. When a cobalt-ferrite pinning layer is used, x ispreferably 1, i.e., the cobalt-ferrite is CoFe₂O₄. PreferableBa-ferrites and Sr-ferrites are Ba₁₂O₁₉ and SrFe₁₂O₁₉, respectively.Ba-ferrite and Sr-ferrite have a hexagonal lattice symmetry thatprovides high magnetic anisotropy, unlike cubic ferrites such asCo-ferrite. While the experimental results described below refer toCoFe₂O₄ as the pinning layer, it is to be understood that the resultsare for illustration purposes only and in no way limit the scope of thepresent invention.

Transmission electron microscope (TEM) cross-sections of two AP-pinnedspin valves without and with a 17-nm Co-oxide underlayer are shown inFIGS. 4A and 4B, respectively. The pinning layer in both cases is a 3-nmthick CoFe₂O₄ layer. In FIG. 4A, the CoFe₂O₄ pinning layer can easily beidentified on the silicon substrate. In FIG. 4B, the CoFe₂O₄ layer isindistinguishable from the Co-oxide underlayer because of low contrastbetween the two layers. From lattice constant considerations, the top˜12 nm of the Co-oxide underlayer can be identified as Co₃O₄. The TEMcross-section in FIG. 4B shows a high amount of crystallinity in theCo-oxide and CoFe₂O₄ layers. Much less crystalline order is observed inFIG. 4A, in which the CoFe₂O₄ layer is grown without an underlayer. Ahigh-resolution image of the spin valve with Co-oxide underlayer isshown in FIG. 4C. From this image it can be inferred that the ferritegrows pseudo-epitaxially on the Co-oxide grains.

The TEM cross-sections (along with data presented below) indicate thatthe enhanced coercivity and improved thermal stability of thin CoFe₂O₄films grown onto CoO or Co₃O₄ is of structural rather than magneticorigin. Therefore the enhanced coercivity and improved thermal stabilitycan also be observed for coercive ferrites grown onto other underlayerswith chemical structure similar to CoO or to Co₃O₄. For information onthe chemical structure of oxides, see J. R. Smyth et al., “ComparativeCrystal Chemistry of Dense Oxide Minerals,” chapter 9 in ComparativeCrystal Chemistry, Reviews in Mineralogy, vol. 40 (2000), hereinincorporated by reference.

In particular, CoO has a crystalline structure that is referred to as arock salt type lattice structure, so named because it is the structureof NaCl (rock salt). The rock salt structure is a cubic close packed(face-centered cubic) anion lattice, with cations in the octahedralholes. The coordination number (number of nearest neighbors of oppositecharge) of each cation and each anion is six. A unit cell contains fourformula units of CoO. In the present invention, similar effects asobserved for CoO can be observed for CoFe₂O₄ grown onto otherunderlayers with a chemical structure of rock salt and with similarlattice constant to that of CoO.

Thus the present invention can be implemented with any underlayer havinga rock salt type lattice structure with similar lattice constant to thatof CoO. The underlayer preferably exhibits no magnetization at roomtemperature, i.e., is neither ferromagnetic nor ferrimagnetic. Forexample, suitable underlayers include ZO_(1+x), where −0.3≦x≦0.3 and Zis Co, Ni, Mg, Mn, or their alloys. Note that excess oxygen (i.e., x≧0)can be introduced into the alloy without changing the crystal structureof the underlayer.

Co₃O₄ has a spinel crystalline structure. Spinels have a general formulaXY₂O₄, where X and Y are cations with variable valence. Oxygen atoms arearranged in cubic closest packing along (111) planes of the structure.The cations are interstitial to the oxygen framework and in octahedraland tetrahedral coordination with oxygen. In a unit cell of spinel,there are 32 possible octahedral sites and 64 possible tetrahedralsites; of these, 16 octahedral and 8 tetrahedral sites are occupied bycations. In a normal spinel, the X cations occupy the eight tetrahedralsites and the Y cations occupy the 16 octahedral sites. In an inversespinel, 8 of the 16 Y cations occupy the tetrahedral sites. In a mixedspine, the Y cations occupy the tetrahedral and octahedral sitesrandomly.

For the present invention, any underlayer can be used that exhibits nomagnetic moment at room temperature and that has the chemical structureof a normal, inverse, or mixed spinel with a similar lattice constant tothat of Co₃O₄. For example, suitable underlayers include Co₃O₄, MgAl₂O₄or alloys of Co₃O₄ or MgAl₂O₄.

The following data describe simple and AP-pinned spin valves that haveCoFe₂O₄ pinning layers and CoO and Co₃O₄ underlayers. As shown by thedata, the addition of CoO or Co₃O₄ underlayers significantly improvesthe stability of thin CoFe₂O₄ pinning layers, making them suitable fornarrow read-gap applications. In fact, the oxide underlayers make itpossible to use cobalt-ferrite pinning layers as thin as 1 nm, asignificant improvement from the tens of nanometers required without anunderlayer. The overall oxide thickness is approximately 30 nm. Themetallic layers, pinned layer, conduction layer, free layer, and cappinglayer are approximately 17 nm. Since the underlayer and the ferrite areinsulating, reducing the thickness for the alumina substrate, it ispossible to fit the sensor into a gap as thin as 50 nm.

CoO, Co₃O₄, and CoFe₂O₄ films were grown by DC magnetron sputtering from2-inch planar targets in a UHV sputtering system under an argon/oxygengas mixture at a pressure of 2 mTorr. The typical base pressure of thesystem was approximately 10.sup.-8 Torr. The CoO and Co₃O₄ underlayerswere sputtered from a cobalt target at 200 mA and 100 mA, respectively,with 45% oxygen in the argon sputtering gas. The CoFe₂O₄ films weresputtered from a CO_(0.33)Fe_(0.67) target at 200 mA with 50% oxygen.After deposition of the cobalt-oxide and cobalt-ferrite layers, thesystem was pumped for 30 min to purge the oxygen. The subsequentmetallic layers were DC-magnetron sputtered at a pressure of 2 mTorr ofargon. All layers were deposited at room temperature. After deposition,the spin valves were annealed in a 10 kOe magnetic field at 200 degreeC. for 1 hour.

Alternatively, the layers can be deposited by any other suitabledeposition method. For example, they can be deposited by ion-beam,pulsed laser, or chemical vapor deposition using techniques known in theart.

The chemical phases of the cobalt-oxides were confirmed by radial X-rayspectra, shown in FIG. 5. The position of the peaks for both CoO (top)and Co₃O₄ (bottom) are consistent with their powder diffraction files,numbers 78-0431 and 74-2120, respectively. The CoO (200) and Co₃O₄ (111)peaks are in unique positions. These peaks, together with other peaks inthe spectrum, identify CoO and Co₃O₄.

FIG. 6 is plot of coercivity (H_(c)) as a function of underlayerthickness for simple spin valves containing CoO and Co₃O₄ underlayers.Coercivity values are obtained from magnetoresistance curves(magnetoresistance vs. applied field) of the spin valve. The structureof this spin-valve, represented as underlayer/pinning layer/pinnedlayer/conductive layer/free layer/capping layer, is: underlayer (xnm)/CoFe₂O₄ (25 nm)/Co_(0.84)Fe_(0.16) (2 nm)/Cu (2.5nm)/Co_(0.84)Fe_(0.16) (2 nm)/Ni₈₀Fe₂₀ (4 nm)/Ru (3 nm). For both CoOand Co₃O₄, the coercivity increases monotonically with underlayerthickness, from 300 Oe for no underlayer to 1000 Oe for a 25-nm Co₃O₄underlayer and 800 Oe for a 25-nm CoO underlayer. For a given underlayerthickness, the coercivity is equal or higher for Co₃O₄ than for CoO. Inboth cases, however, the coercivity is substantially greater thanwithout the oxide underlayer.

FIG. 7 is a plot of spin valve magnetoresistance ratio, ΔR/R, as afunction of underlayer thickness for both CoO and Co₃O₄ underlayers inthe simple spin valves of FIG. 6 (25-nm pinning layer). AR/R measuresthe change in resistance of the device between parallel and antiparallelorientation of the magnetizations of the free and pinned ferromagneticlayers. In general, it is desirable for MR sensors to have a large valueof ΔR/R. As shown in the plot, the magnetoresistance values increasefrom ˜7.3% for no underlayer to ˜10.5% with a 25 nm CoO underlayer. Themagnetoresistance values for Co₃O₄ remain approximately constant as theunderlayer thickness is increased.

Since ferrites exhibit a magnetic moment, it is advantageous to growanti-parallel (AP)-pinned spin valves to balance the overall moment ofthe device. For spin valves, magnetoresistance curves are plotted asmagnetoresistance versus applied magnetic field. The magnetoresistancecurves of an AP-pinned spin valve exhibit two critical magnetic fields,H⁺ and H⁻, which originate from the reversal of theantiferromagnetically coupled pinned and reference layers fromantiparallel to parallel alignment. If the device is operated in fieldsthat exceed either H⁺ or H⁻, it becomes unstable, seen as hysteresis inthe magnetoresistance curves. FIG. 8 is a magnetoresistance curve of anAP-pinned spin valve with critical fields H⁺ and H⁻ indicated. Thestructure of this spin valve, represented as underlayer/pinninglayer/pinned layer/coupling layer/reference layer/conductive layer/freelayer/capping layers, is Co₃O₄ (10 nm)/CoFe₂O₄(10 nm)/CO_(0.84)Fe_(0.16)(2 nm)/Ru (0.8 nm)/Co_(0.84)Fe_(0.16) (1.5 nm)/Cu (2.5nm)/Co_(0.84)Fe_(0.16) (0.5 nm)/Ni₈₀Fe₂₀ (4 nm)/Ru (3 nm). H⁺ and H⁻ aredefined by the magnetic fields at which the tangents to themagnetoresistance curve intersect each other. The coercivity, H_(c), isdefined as the field at which the magnetoresistance ratio drops to halfof its maximum value.

Magnetoresistance curves were obtained for the same AP-pinned spin valveat a series of temperatures to determine the variation of H⁺ and H⁻ withtemperature, plotted in FIG. 9. H⁺ and H⁻ decrease almost linearly withtemperature, with H⁺ greater than H⁻ at all temperatures. The blockingtemperature, T_(b), is defined as the temperature at which H⁻ is zero.The spin valve is unstable at temperatures exceeding T_(b), and so it isdesirable that the blocking temperature be greater than standardoperating temperatures of the device. For the AP-pinned spin valve inFIGS. 8 and 9, T_(b) is approximately 270 degrees C. At 130 degrees C.,a typical read head operating temperature, H⁻ is approximately 700 Oeand H⁺ is approximately 1300 Oe. Without an appropriate underlayer,sufficiently high blocking temperatures are achieved only forcobalt-ferrite layers that are tens of nanometers thick.

A minor magnetoresistance curve as a function of applied magnetic fieldfor the AP-pinned spin valve of FIG. 8 at 130 degrees C. is shown inFIG. 10. The exchange bias field varies from −700 to 700 Oe. The loop isvery square and non-hysteretic for fields up to 700 Oe, allowingbistable operation of the sensor within these field limits at 130degrees C. Note that the magnetoresistance ratio ΔR/R drops to 3.7% atthis temperature.

FIG. 11 shows magnetoresistance curves of AP-pinned spin valves with a3-nm Co-ferrite thickness and different CoO underlayer thicknesses. Allplots show H⁺ curves higher than H⁻ curves. The spin valve structure isCoO (x nm)/CoFe₂O₄ (3 nm)/Co (3 nm)/Ru (0.7 nm)/Co (2 nm)/Cu (2.5 nm)/Co(0.5 nm)/Ni₈₀Fe₂₀ (4 nm)/Ru (3 nm). H⁻ is zero in the absence of a CoOunderlayer but increases to almost the same value as H⁺ as the thicknessof the CoO underlayer increases, showing the increase in devicestability with increasing underlayer thickness.

The stability of the devices with underlayers to transverse fields, suchas might be encountered when setting the direction of longitudinal biaswith an external field, was tested by measuring a minor loop aftersuccessively applying magnetic fields of increasing magnitude in thetransverse direction, that is, the in-plane magnetically hard axis ofthe pinned layer. FIG. 12 plots the magnetoresistance as a series oftransverse fields are applied to an AP-pinned spin valve with thefollowing structure: CoO (17 nm)/CoFe₂O₄ (3 nm)/Co (2.4 nm)/Ru (0.7nm)/Co (2 nm)/Cu (2.5 nm)/Co (0.5 nm)/Ni₈₀Fe₂₀ (4 nm)/Ru (3 nm).Transverse fields are applied in approximately 500 Oe increments. Themagnetoresistance ratio stays constant at ˜7% with increasing fields upto about 5 kOe, after which the device becomes unstable, indicated by adecrease in magnetoresistance ratio. The two insets show minor loopsobtained after applying 2.5 kOe and 7.5 kOe fields. The loop afterapplying a 7.5 kOe field exhibits a typical overshoot, indicatinginstability of the pinned layer, while the loop after applying a 2.5 kOefield still exhibits high squareness. These spin valve are thereforeconsiderably more stable than spin valves with cobalt-ferrite pinninglayers without CoO underlayers, in which reversal begins at fields aslow as 1.5 kOe.

FIGS. 6-12 clearly illustrate that a very thin layer of cobalt-ferritecan be used as a pinning layer when deposited above a cobalt-oxideunderlayer. The key feature of the pinning layer to fix the moment ofthe pinned layer at room temperature as well as elevated temperaturesand its stability versus transverse fields have been demonstrated.

To quantify the impact of the underlayer on the stability of the CoFe₂O₄pinning layer, CoFe₂O₄ films of various thickness were grown directlyonto Si/SiO_(x) and with a 20 nm CoO underlayer between the pinninglayer and the Si/SiO_(x) substrate. FIGS. 13A and 13B show the magneticmoment per area and coercivity, respectively, of the structure as afunction of CoFe₂O₄ thickness. The samples grown onto the 20-nm CoOunderlayers exhibit both higher moment and higher coercivity than thosewithout the underlayer for all CoFe₂O₄ thicknesses. This behavior(combined with the TEM images of FIGS. 4A-4C) suggests that fewer grainsorder in the CoFe₂O₄ phase without an underlayer. The coercivity of thefilms with the CoO underlayer increases steeply from 0.7 kOe for a 3 nmCoFe₂O₄ film to .about.4.0 kOe for all CoFe₂O₄ films thicker than 10 nm.In sharp contrast, the films without an underlayer exhibit no coercivityup to 10 nm, and even for a 60 nm film, the coercivity is more than 1kOe smaller than the film with an underlayer.

While the exchange-coupled magnetic structure of the invention has beendescribed in only simple and AP-pinned spin valve MR sensors, it can beimplemented in all of the MR sensors described in U.S. patentapplication Ser. No. 09/755,556, filed Jan. 4, 2001, (issued as U.S.Pat. No. 6,721,144 to Carey, et al. Apr. 13, 2004) which wasincorporated by reference above. For example, the underlayer can be usedin spin valves containing AP-free structures or extra AFM layers betweenthe pinning and pinned layers.

Second Embodiment—Hard Biasing for Spin-Valves and Magnetic TunnelJunctions

This embodiment of the invention relates to exchange-coupled structuressuch as spin valves and magnetic tunnel junctions (MTJs). MTJs have beenproposed for magnetic memory cells (MRAM) and magnetoresistive readheads. A magnetic tunnel junction consists of two ferromagnetic layersseparated by an insulating non-magnetic tunneling barrier. The barrieris thin enough that quantum-mechanical tunneling occurs between theferromagnetic layers. Since the tunneling probability is spin-dependent,the tunneling current is a function of the relative orientation of thetwo magnetic layers. Thus a MTJ can serve as a MR sensor. For a constantapplied voltage, the resistance of the MTJ changes from a low to a highstate as the relative orientation of the two ferromagnetic layerschanges. Depending upon the electronic band structure of the twoferromagnetic layers, either parallel or antiparallel alignment of theferromagnetic layers defines the high or the low state of the MTJ.

In order to obtain a linear response of a spin valve or magnetic tunneljunction MR sensor, the magnetization of the free layer must be orientedperpendicular to the magnetically pinned layer in the absence of anexternal signal field. In addition, a biasing field is applied throughan external hard ferromagnetic layer with a remanent moment that isseveral times the saturation moment of the sense (free) layer. Withoutthis hard-biasing layer, the magnetic moments in the free layer tend toestablish a multi-domain state, leading to highly undesirable domainreorientation phenomena in the presence of external magnetic fields.

In order to use a MTJ as a magnetoresistive sensor, bistability isnecessary. One solution is shown in FIG. 14, a cross-sectional view of alongitudinal biased magnetic tunnel junction 150 for use in a MR sensorof a read head or a magnetic memory cell. MTJ 150 consists of a pinnedferromagnetic layer 153 and a free ferromagnetic layer 155 separated byan insulating tunnel barrier 154. A pinning layer 152 fixes the magneticmoment of pinned ferromagnetic layer 153. A capping layer (not shown) istypically added to prevent corrosion. A hard-biasing layer 161 sets thedirection of the free layer magnetization via magnetostatic interaction.A top shield and lead 162 and bottom shield and lead 163 contact freeferromagnetic layer 155 and pinning layer 152, respectively, to supplysense current.

MTJ 150 also contains at least one non-magnetic, insulating underlayer151 that controls the growth of hard-biasing layer 161. For the presentinvention, hard-biasing layer 161 is a coercive ferrite pinning layersuch as CoFe₂O₄, and underlayer 151 is an oxide underlayer with a rocksalt or spinel lattice structure. A full description of potentialmaterials for hard-biasing layer 161 and undedayer 151 are discussedabove with reference to pinning layer 132 and oxide underlayer 134 ofspin valve MR sensor 130 shown in FIGS. 3A and 3B. As described abovewith reference to spin valves, underlayer 151 improves the coercivityand thermal stability of the hard-biasing layer 161 and thus improvesthe stability of free layer 155 in the MTJ 150.

A current perpendicular-to-the-plane (CPP) spin-valve MR sensor 200 isshown in cross-section in FIG. 15. Sensor 200 is deposited onto a bottomshield and lead 201 and consists of a pinning layer 203, a pinnedferromagnetic layer 204, a conductive layer 205, and a freeferromagnetic layer 206. A hard-bias material 207 is deposited onto amaterial 202 that exhibits no magnetic moment at room temperature.Sensor 200 is terminated with a top shield and lead 208. Since coerciveferrites such as CoFe₂O₄ are insulating ferrimagnets, they can be usedas hard-bias material 207. Insulating material 202 is an underlayeraccording to the present invention, i.e., an oxide with a rock salt orspinal crystalline structure as described in detail above, exhibiting nomagnetic moment at room temperature. Underlayer 202 enhances the thermalstability of coercive ferrite hard-bias layer 207. A simplecurrent-perpendicular to the plane spin-valve 200 is structurallysimilar to a MTJ, but the insulating tunnel barrier is replaced by aconductive layer 205.

To prevent shunting in a CPP spin-valve or MTJ device, the hard-biasingmaterial must be spatially separated from the sensor when a metallicmaterial is used for hard-biasing. However, if an insulating material isused, it can be deposited in direct contact with the sensor. Since thestrength of the magnetostatic interaction is proportional to the remnantmoment of the hard-biasing material and decreases with distance, athinner layer of hard-biasing material can be deposited in contact withthe free layer than spatially separated from the free layer. Theexchange-coupled magnetic structure of the present invention is verywell suited for this application.

Although not shown, this embodiment of the invention can be used insimple current-in-plane (CIP) spin valves such as spin valve 100 ofFIG. 1. It can also be used in antiparallel-pinned and dual spin valves(CIP and CPP).

Third Embodiment: In-Stack Biasing of Spin Valve MR Sensors

Instead of hard-biasing a current-in-plane (CIP) spin valve MR sensorfrom the side, as in FIG. 1, an in-stack-biasing scheme canalternatively be used. In this case, the biasing material of the freelayer is deposited in direct contact with and parallel to the freelayer. One advantage of using in-stack biasing rather than hard-biasingfrom the side is that fewer processing steps are necessary to fabricatethe read head sensor.

FIG. 16 shows in cross section a simple spin valve MR sensor 300 withinsulating in-stack biasing. An analogous scheme can be used forAP-pinned spin valves (not shown). As shown, sensor 300 is grown inreverse of the previously discussed devices. A coercive ferrite in-stackbiasing layer 301 is deposited onto an oxide underlayer 308, followed bya free ferromagnetic layer 302. Biasing layer 301 and oxide underlayer308 are layers of the present invention as described above withreference to pinning layer 132 and oxide underlayer 134 of FIGS. 3A and3B. As described above, biasing layer 301 provides sufficient biasing ata smaller thickness than would be required without oxide underlayer 308.Subsequently deposited layers are a conductive layer 303, a pinnedferromagnetic layer 304, a pinning layer 305, and a capping layer 306.Pinned ferromagnetic layer 304 has its magnetic moment pinnedperpendicular to the magnetic moment of free ferromagnetic layer 302.For a current-in-plane (CIP) geometry, leads 307 contact pinned layer304. For a current perpendicular-to-the-plane (CPP) geometry, a secondset of leads (not shown) is deposited in contact with free layer 302.

It is to be understood that the exchange-coupled magnetic structure ofthe present invention can be used in any suitable device, not only thedevices described in the three embodiments above.

FIG. 17 is a schematic diagram of a disk drive system 400 containing amagnetic recording disk 402 rotated by a motor 404. An actuator 406holds a magnetoresistive read/write head 408 and moves read/write head408 across magnetic recording disk 402. MR read/write head 408 containsa magnetoresistive sensor 410 incorporating at least oneexchange-coupled magnetic structure of the present invention. Rotationof magnetic recording disk 402 and movement of actuator 406 allows MRread/write head 408 to access different regions of magnetically recordeddata on magnetic recording disk 402.

It will be clear to one skilled in the art that the above embodimentsmay be altered in many ways without departing from the scope of theinvention. Accordingly, the scope of the invention should be determinedby the following claims and their legal equivalents.

1. A magnetoresistive (MR) sensor comprising: a) a ferromagnetic layerstructure including first and second ferromagnetic layers separated bynonmagnetic layer; b) a coercive ferrite layer biasing a magnetizationof the ferromagnetic layer structure; and c) an oxide underlayer for thecoercive ferrite layer, the oxide underlayer being deposited prior tothe coercive ferrite layer to influence growth characteristics of thecoercive ferrite layer, the oxide underlayer having a spinel latticestructure and exhibiting no magnetic moment at room temperature.
 2. TheMR sensor of claim 1 wherein the oxide underlayer comprises a materialselected from the group consisting of Co₃O₄, MgAl₂O₄, and their alloys.3. The MR sensor of claim 1 wherein the MR sensor is a spin valve MRsensor.
 4. The MR sensor of claim 3 wherein the second ferromagneticlayer is a pinned layer and the coercive ferrite layer is a pinninglayer that pins the magnetization of the second ferromagnetic layer. 5.The MR sensor of claim 4 wherein the second ferromagnetic layer is afree layer and the coercive ferrite layer is an in-stack biasing layerthat biases the magnetization of the free layer.
 6. The MR sensor ofclaim 1 wherein the sensor is an antiparallel-pinned spin valve MRsensor.
 7. The MR sensor of claim 1 wherein the sensor is a dual spinvalue MR sensor.
 8. The MR sensor of claim 1 wherein the MR sensor is amagnetic tunnel junction MR sensor.
 9. The MR sensor of claim 1 whereinthe coercive ferrite layer comprises a material selected from the groupconsisting of Co_(x)Fe_(3−x)O₄, wherein 0≦x≦1.5, SrFe₁₂O₁₉, BaFe₁₂O₁₉,and their alloys.
 10. The MR sensor of claim 1 wherein the coerciveferrite layer comprises an alloy comprising an element selected from thegroup consisting of Si, Ti, Mg, Al, Mo, Os, Re, Ru, and W.
 11. The MRsensor of claim 1 wherein the coercive ferrite layer comprises CoFe₂O₄.12. The MR sensor of claim 1 wherein the coercive ferrite layer has athickness of between 1 and 30 nm.
 13. The MR sensor of claim 1, furthercomprising an upper shield and a lower shield defining a gap, whereinthe gap contains the ferromagnetic layer, the coercive ferrite layer,and the oxide underlayer and has a width of less than 50 nm.
 14. The MRsensor of claim 1 wherein the coercive ferrite layer is deposited ontothe oxide underlayer.
 15. The MR sensor of claim 1, further comprisingan intermediate layer between the oxide underlayer and the coerciveferrite layer, the intermediate layer being deposited on the oxideunderlayer and preserving the growth characteristics established by theoxide underlayer.
 16. An exchange-coupled magnetic structure comprising:a) a ferromagnetic layer structure including first and secondferromagnetic layers separated by a nonmagnetic layer; b) a coerciveferrite layer, wherein an exchange bias field from the coercive ferritelayer biases the magnetization of the ferromagnetic layer structure; andc) an oxide underlayer for the coercive ferrite layer, the oxideundedayer being deposited prior to the coercive ferrite layer toinfluence growth characteristics of the coercive ferrite layer, theoxide underlayer having a spinel lattice structure and exhibiting nomagnetic moment at room temperature.
 17. The exchange-coupled magneticstructure of claim 16, wherein the second ferromagnetic layer is a freelayer whose magnetization is biased by the coercive ferrite layer, andthe first ferromagnetic is a pinned ferromagnetic layer.